1. Field of Invention
This invention relates to apparatus for inducing local cerebral hypothermia consisting of a stereotaxic interstitial brain cooling probe, introducer sheath, and related accessories.
2. Description of Prior Art
Stroke is a leading cause of death and disability. It is estimated that over 725,000 people suffer a major stroke in the United States each year, and that over 100,000 of these people die. There are two main categories of stroke: ischemic and hemorrhagic. A blockage in an artery in the brain causes ischemic stroke, and a rupture in an artery in the brain causes hemorrhagic stroke. There are approximately 600,000 ischemic stokes, and 125,000 hemorrhagic strokes in the United States each year.
Within the last decade there has been a marked increase in understanding why and how brain cells die from ischemic stroke. Cells within an infarction zone have dramatically reduced blood flow of 20% of normal or less. Cells within this infarction zone will be irreversibly damaged within a few minutes. Surrounding the infarcted zone is a volume of tissue called the “ischemic penumbra” or “transitional zone” in which blood flow is between 20% and 50% of normal. Cells in this area are endangered, but not irreversibly damaged. Ischemia in the infarction zone, and in the ischemic penumbra causes the ischemic cells to release excitatory proteins which migrate into surrounding tissues triggering a hyper metabolic response that leads to cell death beyond the infarction zone and the ischemic penumbra. This hyper metabolic response triggers inflammation, edema, local and global pyrexia, cerebral hypertension, apoptosis, and an increase in intra-cranial pressure causing a cascade of cell injury and death. Nowhere in the art is a mechanism described that can effectively prevent or limit the migration of excitatory proteins from ischemic tissue, to surrounding normal tissue in the brain, and simultaneously resolve the ischemic penumbra.
Hypothermia has long been known to be neuroprotective. There are countless anecdotes where resuscitation of cold water drowning victims has been successfully achieved after an extended period of cardiac arrest with few untoward effects. In the 1970's deep systemic hypothermia (core temperature below 30 Deg. C.) was evaluated as a therapy for stoke and other cerebral indications but was abandoned due severe systemic complications. Since 1990 there has been a significant effort to evaluate mild systemic hypothermia (core temperature of 32 to 34 Deg. C.) as a therapy for stroke in animal models, and more recently in clinical trials.
Mild systemic hypothermia has been shown to significantly reduce infarct size, and improve neurological outcome in animal models.
Recent clinical reports of have indicated that mild systemic hypothermia may have a significant positive benefit for patients suffering severe focal ischemic stroke.
Swartz et. al. recently reported treating 25 patients with severe MCA territory stroke with systemic hypothermia at 33 deg. C. for 48 to 72 hours. 56% of the patients survived; historical controls indicated that a survival rate of less than 20% would normally be expected. This study also demonstrated that mild hypothermia is effective in reducing critical rises in intra-cranial pressure.
At the 26th Annual International Stroke Conference held this past January, Krieger et al reported treating 10 patients with 19 matched controls with systemic hypothermia at 32 Deg. C. for 48 hours. At 3 months 50% of the patients in the treatment arm had a “good” neurological outcome vs. 10% in the control arm. The initiation of a 350 patient randomized study was announced.
There is a large body of evidence in the scientific literature, mostly developed within the past ten years, which demonstrates that hypothermia has a broad spectrum of neuroprotective effects against ischemia.
Hypothermia:
Reduces metabolic rate                Lowers ATP requirements        Reduces lactic acidosis        
Reduces production of excitatory proteins
Reduces edema                Reduces neutrophil accumulation        Reduces glial cell activation        
Stabilizes the blood/brain barrier
Hypothermia's effect on metabolic rate and production of excitatory proteins is temperature dependent, where the lower the temperature, the greater the neuroprotective effect. Hypothermia's effect on edema stabilizes at about 33 Deg. C. with no further benefit at lower temperatures.
Early clinical experience has demonstrated that sustained systemic hypothermia below 30 Deg. C. is impractical. Complications of systemic hypothermia below 30 Deg. C. include cardiac arrhythmia and arrest, hemorrhage due to systemic coagulation disorders, pancreatitis, pneumonia, and death.
Recent clinical experience has demonstrated that sustained systemic hypothermia at 32 to 33 Deg. C. may be practical. The most common reported systemic complication is pneumonia. However, for a patient to tolerate even mild systemic hypothermia, full anesthesia is required for the duration of the therapy. Also, rewarming following hypothermia is considered a critical phase of the therapy. An uncontrollable rise in intra-cranial pressure has been a frequent event during the rewarming phase. The risks of sustained mild hypothermia beyond 72 hours are unknown.
It has long been known that hypothermia causes vasoconstriction in the limbs, and vasodilation in vital organs. This is a basic mammalian survival mechanism where blood is directed to vital organs at the expense of the rest of the body during hypothermic challenge. The brain, and the heart, being the most vital organs, have the greatest capacity for vasodilation during hypothermic challenge.
It has been well documented that even mild systemic hypothermia reduces cerebral blood flow rate. In an animal model, cerebral blood flow rate decreases approximately 5% for each degree centigrade reduction in core temperature.
Kuluz et al demonstrated in a well-understood animal model that selective brain cooling, where the brain is cooled, and the body is maintained at normal temperature results in a significant increase in cortical cerebral blood flow rate. There was a 215% increase in cortical cerebral blood flow when the brain was cooled to 33 Deg. C., with a slight increase in cortical cerebral blood flow when the temperature was further lowered to 30 Deg. C. Cortical cerebral blood flow returned to base line when normal temperature was restored.
Kuluz suggests that hypothermia causes vasodilation, which is maximal at a temperature between 30 and 33 Deg. C. Although both systemic hypothermia and selective cerebral hypothermia both result in vasodilation, the suppressive effect of systemic hypothermia on cardiac output results in a net decrease in cerebral blood flow. When hypothermia is applied selectively to the brain, vasodilation of the blood vessels within the brain results in a decrease in cerebral vascular resistance relative to the rest of the body, resulting in an increase in cerebral blood flow. Kuluz also suggests that the neuroprotective effects of hypothermia may be more related to vasodilation than to biochemical mechanisms.
Intra-parenchymal brain temperature may exceed core body temperature by as much as 2.4 Deg. C. during the acute phase of focal ischemic stroke. Elevated brain temperature during the acute phase of a stroke is presumed to be the result of hyper-metabolic processes caused by ischemia. Although temperature within an infarction or the ischemic penumbra has not been measured during acute ischemic stroke, it is presumed to be at a higher temperature than the average brain temperature, which has been measured. This is based on the premise that excitatory proteins released by the ischemic brain tissue within the infarction and ischemic penumbra causes the rate of metabolic activity within the infarction and the ischemic penumbra to be greater than of the rest of the brain. The hyper-metabolic rate within the infarction and the ischemic penumbra, in combination with diminished blood flow results in a temperature within the infarction and ischemic penumbra, which exceeds the temperature in the rest of the brain.
During systemic hypothermia treatment of focal ischemic stroke, the temperature within the infarction and ischemic penumbra remain at a higher temperature than the rest of the brain and body. During local cerebral hypothermia, by design, the infarction and the ischemic penumbra are at lower temperatures than the rest of the brain.
Since systemic hypothermia is limited by patient tolerance to 32 to 33 Deg. C., and the infarction and ischemic penumbra remain at higher temperatures than surrounding brain tissue, maximal vasodilation effect is not likely to be achieved. Also, the suppressive effect of systemic hypothermia on the release of excitatory proteins from within the infarction and the inner core of the ischemic penumbra are likely to be minimal or non-existent.
Local cerebral hypothermia provides a temperature profile within the affected area of the brain that provides maximum vasodilation within the ischemic penumbra without reducing cardiac output, and provides for significant suppression of excitatory protein release from within the infarction.
There are several examples in the art where catheters are constructed with a cooling means, which is placed into the carotid artery to cool the blood entering the head. This offers an advantage over systemic hypothermia, since it provides a means to cool the head to lower temperatures than the rest of the body, but it still results in systemic hypothermia. Also, since the scientific evidence suggests that hypothermia must be maintained for extended periods of time, there is a great risk that clots will form on the catheters and migrate into the brain leading to further episodes of stroke. The mechanism of cooling a zone of infarction in the brain, or the surrounding transitional zone with this approach is the same as with systemic hypothermia, and does not overcome the significant limitations as described above.
Often, an infarction will hemorrhage spontaneously and lead to a poor outcome for the patient. Spontaneous hemorrhage in an infarction is believed to be caused by a combination of the deterioration of the blood vessels, and a local increase in blood pressure due to ischemia. There is a wide spread perception held by neurosurgeons that if a probe is placed into an infarction in the brain there is a significant risk that the trauma of placing the probe will cause hemorrhage due to the frail nature of the blood vessels in the infarction.
There are numerous examples of interstitial cooling probes in the art. Nowhere in the art is it suggested that interstitial cooling probes may be used to treat stroke, and nowhere in the art is there an example of a cooling probe that may be practically fixated to the head and left indwelling in the brain for the extended periods of time required for effective hypothermia treatment of stroke, that also provides for a means to coagulate the core of an ischemic lesion sufficiently to mitigate the risk of hemorrhage.